Project 2 focuses on the ability of BDNF (brain-derived neurotrophic factor) in the nucleus accumbens (NAc) and its dopaminergic input from the VTA (ventral tegmental area) to regulate mood and motivational state. Previously, we and other groups have focused on BDNF in hippocampus as a possible substrate for antidepressant medications and stress. However, it has been difficult to obtain functional direct evidence for the involvement of this particular brain region in BDNF's behavioral effects. In contrast, we know that BDNF in the VTA and NAc has powerful effects on reward and appetite. Consistent with these data are our preliminary findings that BDNF, acting within the NAc, also exerts potent effects in several animal models of depression in both rats and mice. The goal of the proposed studies is to establish the role played by BDNF in the VTA-NAc pathway as it relates to mood, affect, and motivational state. Mice with conditional knockouts of BDNF, or those with inducible and cell-targeted knockout of BDNF, will be used in these experiments. This work in mice will be complemented by studies of viral-mediated gene transfer and local intra-cerebral infusions of BDNF in rats. A second goal is to begin to identify the molecular substrates through which BDNF produces these effects in the VTA-NAc pathway. We believe that one of these substrates is CREB, given the known role of BDNF as a key regulator of CREB activity in vitro. This possibility will be investigated in vivo, an approach that is now possible given the powerful tools available to this Center. In addition, we are interested in the converse: whether CREB may control the activity of the BDNF system within NAc circuits. BDNF is expressed at low levels in the NAc, where we have shown that its expression is regulated by CREB. BDNF is expressed at much higher levels in the VTA; it is believed that VTA dopamine neurons-via their projections to the NAc-are an important source of BDNF for NAc neurons. We have shown that stress downregulates BDNF expression within the VTA, an effect prevented by antidepressant treatment. Since stress regulates CREB function in both brain regions, our hypothesis is that stress and antidepressant regulation of BDNF expression is mediated via CREB. We hypothesize further that such regulation of BDNF contributes to changes in mood and motivational state seen under these treatment conditions.